1. Field of the Invention
This invention relates to a laser plate making apparatus for making a press plate for a gravure printing press and the press plate made by the laser plate making apparatus.
2. Background of the Invention
FIG. 7 schematically shows a construction of a laser scanning and optical system of a general laser plate making apparatus. In the apparatus shown in FIG. 7, a laser block 4 typically includes a semiconductor laser 1, a collimator lens 2 and an objective lens 3. In opposing relation to the laser block 4, there is disposed a plate cylinder 6 around an outer surface of which a form plate sheet 5 is wound. Then, a laser light emitted from the semiconductor laser 1 is made into a parallel light by the collimator lens 2, and then it is converged by the objective lens 3 and incident upon the form plate sheet 5 on the plate cylinder 6 as a laser beam L.
In this case, for example, the laser beam L controlled in its on/off state is irradiated on the form plate sheet 5 while the plate cylinder 6 around which the form plate sheet 5 is wound is rotated at a constant rate in the direction of arrow E (the opposite direction to a main scanning direction F of the laser beam L), whereby a recess (hereinafter referred to as a dot upon necessity) 7 corresponding to image information is formed on the form plate sheet 5.
After the recesses 7 are formed on one line in the main scanning direction F, the laser block 4 is fed in a step feeding manner by an amount of one line in the direction of arrow A (sub scanning direction) which is the axial direction of the plate cylinder 6 by a feeding mechanism not shown, so that the recesses 7 are formed on a new line.
FIG. 8A shows an example of a recess of a fundamental shape (hereinafter referred to as a fundamental recess or simply, recess upon necessity) 7A of the recess 7. That is, the fundamental recess 7A of the recess 7 has a substantially rectangular shape corresponding to a beam profile of the laser beam L converged on the form plate sheet 5 by the objective lens 3. Its shape is, for example, such that its length dH is dH=120 .mu.mm and its width dV is dV=0.6 .mu.mm. Further, if the direction of the length dH of the recess 7 formed on the form plate sheet 5 is a direction parallel to the axial direction of the plate cylinder 6, the recess 7 is one formed under the condition that the irradiation angle .theta. of the laser beam L is .theta.=0.degree. (horizontal direction).
Then, the form plate sheet 5 is rotated in the direction of arrow E, during which if the semiconductor laser 1 is in on-state there are formed recesses 7B, 7C or the like in which the width dV shown in FIG. 8A is elongated as shown in FIG. 8B and FIG. 8C by a time corresponding to the duration of the on-time.
Finally, the form plate sheet 5 has formed thereon image information formed of halftone dots differing in areas by the recesses 7. In addition, if gradation is further increased, the amount of radiation of the laser beam L may be increased or decreased to control the depth of the recesses 7.
Then, when the recesses 7 are sequentially formed all over the main scanning lines, a plane scanning relative to the form plate sheet 5 is finished and a press plate 5P (the form plate sheet 5 on which the plane scanning is finished) is completed.
If a plurality of, for example, four press plates 5P thus manufactured are utilized to carry out a multicolor printing of four color printing or the like such as of C (cyan) color, M (magenta) color, Y (yellow) color and K (black) color, four press plates 5P for respective colors are manufactured and the four press plates 5P are loaded on a press machine. Then, an ink of each color is coated on the press plate 5P for each color, one sheet of paper as a printed material is sequentially pressed against each press plate 5P of which recesses 7 are filled with ink, and the ink is transferred to the sheet of paper overlaid thereon, whereby a printed material of a multicolor printing (hard copy) is produced.
Additionally, if the multicolor printing is carried out by using the press plate 5P having formed thereby an image information formed of the halftone dots by recesses 7 which are formed of a single kind of fundamental recesses 7A, undesirable stripe patterns of shading having various shapes known as Moire are formed.
As a technology for suppressing the Moire from being caused, the present applicant has proposed a technology which was published in Japanese Patent publication gazette No. 4-67950.
FIG. 9 shows a mechanical construction of the laser block 4 which is used for implementing the technology and has been published in the above publication gazette.
In the laser block 4, a laser holder supporting box 18 is fixed to a laser head mounting stage 11 having a substantially T-letter shape and also a plate 12 is fixed thereto. Then, a stepping motor 13 is fixed to the plate 12. The stepping motor 13 has mounted on its rotary shaft a first gear which does not appear in the figure for purposes of clarity.
The first gear is meshed with a second gear 14. A semiconductor laser 1 is fixed coaxially and integrally with the shaft of the second gear 14 through a semiconductor laser holder 15. In addition, a laser holder, a collimator lens, though not shown, and an objective lens holder 16, an objective lens 3 are fixed coaxially and integrally with the shaft of the second gear 14 in an emitting direction of the semiconductor laser 1, or the optical axis direction.
In the above construction, the second gear 14 can be rotated in the direction of arrow G or the direction of arrow H by driving the stepping motor 13 through the above first gear. When the semiconductor laser 1 is rotated by the rotation of the second gear 14, an irradiation angle .theta. of the laser beam L can be changed and a dot pattern as a countermeasure against Moire can be formed on the press plate 5P.
FIG. 10A to FIG. 10F show examples of dot patterns as a countermeasure against Moire which are formed on the press plate 5P by using the laser block 4 of the example in FIG. 9.
FIG. 10E is an example of a press plate 5P.sub.o in which the recesses 7 are formed at an irradiation angle of .theta.=0.degree..
FIG. 10F is an example of a press plate 5P.sub.ow in which a distance "a" in the sub scanning direction A between the adjacent recesses 7 formed at the irradiation angle of .theta.=0.degree. is widened as compared with the example in FIG. 10E.
FIG. 10A shows an example of a press plate 5P.sub.-30 under the condition that the semiconductor laser 1 is rotated by 30.degree. in the counterclockwise direction (in terms of notation, it is referred to as -30.degree.) with respect to the sub scanning direction A so as to make the irradiation angle .theta., .theta.=-30.degree..
Similarly, FIG. 10B shows a press plate 5P.sub.30 formed under the condition that the semiconductor laser 1 is rotated by .theta.=30.degree. in the clockwise direction with respect to the sub scanning direction A, FIG. 10C shows a press plate 5P.sub.60 formed under the condition that it is rotated by .theta.=60.degree. in the counterclockwise direction with respect to the sub scanning direction A, and FIG. 10D shows a press plate 5P.sub.60 formed under the condition that it is rotated by .theta.=-60.degree. in the clockwise direction with respect to the sub scanning direction A, respectively.
Of the plurality of press plates 5P thus manufactured, for example, the press plate 5P.sub.-30 (FIG. 10A) is used for a C-color plate, the press plate 5P.sub.30 (FIG. 10B) is used for a M-color plate, and the press plate 5P.sub.OW (FIG. 10F) is used for a Y-color plate and a K-color plate and then multicolor printing is carried out. As a result, Moire is suppressed to be almost not conspicuous on the produced printed material.
In this prior art, it should be noted that in each of the dot patterns shown in FIG. 10A to FIG. 10F, a slant line R formed of the starting position of the arbitrary recess 7a in the main scanning direction F is coincident with a slant line R formed of the starting position in the main scanning direction F of the recess 7b adjacent to that recess in the sub scanning direction A. It is noted that in the description of the specification or the drawings, the slant line R also includes a case of the irradiation angle .theta.=0.degree., as shown in FIG. 10E.
However, according to this prior art, the rotation mechanism of the laser block 4 of the example in FIG. 9 for changing the irradiation angle .theta. of the semiconductor laser 1 is complicated and the apparatus itself becomes expensive. In addition, if the central axis of the rotary mechanism is not accurately coincident with the optical axis of the optical system, blur or the like is caused upon overlaying the colors, which undesirably degrades the quality of the picture printed and formed on the printed material.
This invention is made in consideration of the above problem with an object to provide a laser plate making apparatus having a simple construction capable of reducing the manifestation of Moire.